X-ray imaging has long been an accepted medical diagnostic tool. X-ray imaging systems are commonly used to capture, as examples, thoracic, cervical, spinal, cranial, and abdominal images that often include information necessary for a doctor to make an accurate diagnosis. X-ray imaging systems typically include an X-ray source and an X-ray sensor. When having a thoracic X-ray image taken, for example, a patient stands with his or her chest against the X-ray sensor as an X-ray technologist positions the X-ray sensor and the X-ray source at an appropriate height. X-rays produced by the source travel through the patient's chest, and the X-ray sensor then detects the X-ray energy generated by the source and attenuated to various degrees by different parts of the body. An associated control system obtains the detected X-ray energy from the X-ray sensor and prepares a corresponding diagnostic image on a display.
The X-ray sensor may be a conventional screen/film configuration, in which the screen converts the X-rays to light that exposes the film. The X-ray sensor may also be a solid state digital image detector. Digital detectors afford a significantly greater dynamic range than conventional screen/film configurations, typically as much as two to three times greater.
One embodiment of a solid state digital X-ray detector may be comprised of a panel or array of semiconductor field-effect transistors (FETs) and photodiodes. The FETs and photodiodes in the panel are typically arranged in rows (scan lines) and columns (data lines). A FET controller controls the order in which the FETs are turned on and off. The FETs are typically turned on, or activated, in rows. When the FETs are turned on, charge to establish the FET channel is drawn into the FET from both the source and the drain of the transistor. The source of each FET is connected to a photodiode. The drain of each FET is connected to readout electronics via data lines. Each photodiode integrates the light signal and discharges energy in proportion to the X-rays absorbed by the detector. The gates of the FETs are connected to the FET controller. The FET controller allows signals discharged from the panel of photodiodes to be read in an orderly fashion. The readout electronics convert the signals discharged from photodiodes. The energy discharged by the photodiodes in the detector and converted by the readout electronics is used by an acquisition system to activate pixels in the displayed digital diagnostic image. The panel of FETs and photodiodes is typically scanned by row. The corresponding pixels in the digital diagnostic image are typically activated in rows.
The FETs in the X-ray detector act as switches to control the charging and discharging of the photodiodes. When a FET is open, an associated photodiode is isolated from the readout electronics and is discharged during an X-ray exposure. When the FET is closed, the photodiode is recharged to an initial charge by the readout electronics. Light is emitted by a scintillator in response to X-rays absorbed from the source. The photodiodes sense the emitted light and are partially discharged. Thus, while the FETs are open, the photodiodes retain a charge representative of the X-ray dose. When a FET is closed, a desired voltage across the photodiode is restored. The measured charge amount to re-establish the desired voltage becomes a measure of the X-ray dose integrated by the photodiode during the length of the X-ray exposure.
X-ray images may be used for many purposes. For instance, internal defects in a target object may be detected. Additionally, changes in internal structure or alignment may be determined. Furthermore, the image may show the presence or absence of objects in the target. The information gained from X-ray imaging has applications in many fields, including medicine and manufacturing.
In any imaging system, X-ray or otherwise, image quality is of primary importance. In this regard, X-ray imaging systems that use digital or solid state image detectors (“digital X-ray systems”) face certain unique difficulties. Difficulties in a digital X-ray image could include image artifacts, “ghost images,” or distortions in the digital X-ray image. One source of difficulty faced by digital X-ray systems is the non-ideal characteristics of semiconductor devices used in the digital X-ray systems.
Ideally, FET switches isolate the photodiode from the electronics which restore the charge to and measure the charge upon the photodiode. In reality, FETs do not completely isolate the photodiode from the system, when the FETs are open. Consequently, under certain circumstances the FETs transfer excess charge to the readout electronics. The unintended charge leakage through the FETs may produce artifacts. Another source of difficulties is unintended charge generated in the panel as a result of electromagnetic fields generated from external sources.
FETs and other materials made of amorphous silicon also exhibit a characteristic referred to as charge retention. Charge retention is a structured phenomenon and may be controlled to a certain extent. Charge retention corresponds to the phenomenon whereby not all of the charge drawn into the FET to establish a conducting channel is forced out when the FET is turned off. The retained charge leaks out of the FET over time, even after the FET is turned off, and the leaked charge from the FET adds an offset to the signal read out of the photodiodes by the X-ray control system.
The FETs in the X-ray detector exhibit charge retention characteristics when voltage is applied to the gates of the FETs to read the rows of the X-ray detector. The detector rows are generally read in a predetermined manner, sequence, and time interval. The time interval may vary between read operations for complete frames of the X-ray image. When a FET is opened after the charge on an associated photodiode is read by a charge measurement unit, the FET retains a portion of the charge. Between read operations, the charge retained by the FETs leaks from the FETs to a charge measurement unit. The amount of charge that leaks from the FETs exponentially decays over time. The next read operation occurs before the entire retention charge leaks from the FETs. Consequently, the charge measurement unit measures during each read operation an amount of charge that was stored by the FETs during the previous read operation. The charge measurement unit also reads an amount of charge that was stored by FETs that were activated in scan lines preceding the current scan line in the current read operation.
The charge remaining on the FETs when a new read operation is initiated is referred to as the initial charge retention. The initial charge retention stored on multiple FETs, such as the FETs of a single data line, combines to form a charge retention offset for that column. The charge retention offset varies based on the rate at which rows of the X-ray detector panel are read. As the interval increases between read operations, the charge decay increases. As the panel rows are read, the charge retention offset builds to a steady state value. The steady state value for the charge retention rate represents the point at which the panel rows are read at a rate equaling the exponential decay rate of the charge on the FETs.
If the times between frames for both the offset and X-ray image are consistent, the effect of charge retention may be eliminated from the final image. In the normal process of reading a detector, the effect of retained charge may be minimized by simply subtracting the results of a “dark” scan from the results of an “exposed” scan. A “dark” scan is a reading done without X-ray. A “dark” scan simply activates the FETs on the X-ray detector panel. Thus, a “dark” scan may determine the charge retention characteristics exhibited by the FETs activated to read the X-ray detector. By subtracting the “dark” scan from the actual “exposed” scan of a desired object, the charge retention effects may be eliminated.
During an X-ray exposure, a similar phenomenon occurs whereby charge is generated in the FET. When the FETs are turned off at the end of the exposure, the additional charge also leaks out and adds to the read signal in a manner analogous to charge retention. However, the additional charge cannot be removed because the additional charge relates to the X-rays bombarding the X-ray detector. Thus, the additional charge is not predictable or nor is it reproducible in a “dark” image where no X-rays are transmitted. The number of FETs affected and the amount of charge conducted by the FETs are dependent upon the amount of X-ray exposure and the object imaged, as well as upon the individual properties of each FET. Since a solid state X-ray detector is structured along rows (scan lines) and columns (data lines), the excess charge in the FETs may result in structured image artifacts or offsets which cannot be corrected by contrasting the “exposed” image with a “dark” image.
Fortunately, correction mechanisms can be used to remove the artifacts caused by unintended charge. However, the correction of the unintended charge can have the effect of increasing the time to read an image and can also ironically have the counterproductive effect of adding noise to the image. Often, the benefits of the correction outweigh the detriments. However, it is desirable to only apply the correction when it is warranted. The correction for unintended charge may occur in addition to normal offset correction.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art to reduce indiscriminate correction of unintended charge from a digital X-ray detector, thereby reducing the number of images with increased noise and read time by the digital X-ray detector.